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1 

Proceedings of 2009 ASME Joint Rail Conference 

JRC2009 

March 3-5, 2009 Pueblo, Colorado, USA 

JRC2009-63025 

IMPROVED TANK CAR DESIGN DEVELOPMENT: ONGOING STUDIES ON 

SANDWICH STRUCTURES 

David Y. Jeong 

David C. Tyrell 

Michael E. Carolan 

A. Benjamin Perlman 

Volpe National Transportation Systems Center 

US Department of Transportation 

Cambridge, Massachusetts, USA 

ABSTRACT 

The Government and industry have a common interest in 

improving the safety performance of railroad tank cars carrying 

hazardous materials. Research is ongoing to develop strategies 

to maintain the structural integrity of railroad tank cars carrying 

hazardous materials (hazmat) during collisions. 

This paper describes engineering studies on improved tank 

car concepts. The process used to formulate these concepts is 

based on a traditional mechanical engineering design approach. 

This approach includes initially defining the desired 

performance, developing strategies that are effective in meeting 

this performance, and developing the tactics for implementing 

the strategies. The tactics are embodied in the concept. The 

tactics and concept evolve through engineering design studies, 

until a design satisfying all of the design requirements is 

developed. Design requirements include service, 

manufacturing, maintenance, repair, and inspection 

requirements, as well as crashworthiness performance 

requirements. 

One of the concepts under development encases the 

pressurized commodity-carrying tank in a separate carbody. 

Moreover, this improved tank car concept treats the pressurized 

commodity-carrying tank as a protected entity. Welded steel 

sandwich structures are examined as a means to offer 

protection of the commodity tank against penetrations from 

impacting objects in the event of a collision. Sandwich 

structures can provide greater strength than solid plates of 

equal weight. Protection of the tank is realized through 

blunting of the impacting object and absorption of the collision 

energy. Blunting distributes impact loads over a larger area of 

the tank. Energy absorption reduces the demands on the 

commodity tank in the event of an impact. In addition, the 

exterior carbody structure made from sandwich panels is 

designed to take all of the in-service loads, removing the 

commodity tank from the load path during normal operations. 

Design studies described in this paper focus on the 

protection aspect of using sandwich structures. Studies are 

conducted to investigate the influence of different parameters, 

such as sandwich height and core geometry, on the forcedeformation behavior of sandwich structures. Calculations are 

carried out numerically using nonlinear finite element analysis. 

These analyses are used to examine the crashworthiness 

performance of the conceptual design under generalized impact 

scenarios. 

INTRODUCTION 

Over the past several decades, Government and industry 

have sponsored research to improve the safety performance of 

railroad tank cars. However, several recent accidents involving 

the release of hazardous material (hazmat) have focused 

attention on the safety performance of railroad tank cars during 

accidents such as derailments and collisions. Table 1 lists 

recent railroad hazmat accidents that have been or are being 

investigated by the National Transportation Safety Board 

(NTSB) for their probable cause. In some accidents, release of 

hazardous materials occurred from an impacting object (e.g., 

broken coupler) penetrating and puncturing the end (or head) 

or the side (or shell) of the commodity-carrying tank. 

Results from industry-sponsored research are applied to 

develop and implement tank car designs to improve safety 

performance. An industry research and development effort 

began in 2006, called the Next-Generation Rail Tank Car 

(NGRTC) project, specifically for this purpose. Dow Chemical 

Company, Union Pacific Railroad, and Union Tank Car 

Company were the industry sponsors of this project. In 2007, 

the Federal Railroad Administration (FRA) and Transport 

Canada, two Government regulatory agencies in North 
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2 

America, began collaborating with the NGRTC project through 

Memoranda of Cooperation. Results from Governmentsponsored research are used to support rulemaking [1]. 

Cooperative activities include full-scale shell impact testing of 

conventional tank cars. Separate activities include design 

development. 

Table 1: Recent Railroad Hazmat Accidents 

Location Date 

Minot, North Dakota January 18, 2002 

Macdona, Texas June 28, 2004 

Graniteville, South Carolina January 6, 2005 

Anding, Mississippi July, 10, 2005 

Texarkana, Arkansas October 15, 2005 

New Brighton, Pennsylvania October 20, 2006 

Shepherdsville, Kentucky January 16, 2007 

Oneida, New York March 12, 2007 

The objectives of FRA-sponsored hazmat research are: (1) 

to understand the load and deformation environment under 

which tank cars operate during normal operations and during 

accidents, (2) to understand structural performance of tank cars 

in the current fleet, (3) to develop alternative designs, operating 

procedures and technologies for maintaining tank integrity 

during all scenarios, and (4) to assist the FRA Office of Safety 

in the promulgation of responsive rules to maintain the safety 

of hazardous materials transportation by rail. In addition, 

meetings with the industry are being planned to facilitate 

stakeholder involvement. 

This paper describes FRA-sponsored work in progress on 

the development of improved crashworthiness tank car designs. 

Previous work on the improved design development is 

reviewed, which includes the overall approach in developing an 

improved design [2] and the preliminary conceptual design 

developed from that approach [3]. Moreover, this paper 

examines the evaluation of welded steel sandwich structures as 

a means to protect the commodity tank against penetrations 

from impacting objects in the event of a collision. 

IMPROVED DESIGN DEVELOPMENT APPROACH 

The Volpe National Transportation Systems Center (Volpe 

Center) conducts and manages research to support the FRA in 

its mission to promulgate rail safety regulations. The objective 

of this research is to maintain the integrity of the commoditycarrying tank under a broad range of loading conditions that 

vary from the normal operating environment to rare events such 

as accidents. As a consequence of the recent railroad hazmat 

accidents, the structural integrity and crashworthiness of 

railroad tank cars has come under great scrutiny. Moreover, 

recent research has focused primarily on maintaining tank 

integrity under accident loading conditions. The framework for 

improved tank car design development effort was described in 

previous work [2]. 

Processes of mechanical engineering design provide the 

basis of FRA/Volpe Center research to formulate concepts for 

improved tank car designs. In the present context, mechanical 

engineering design refers to the utilization of mathematics, 

material science, and engineering mechanics principles to 

develop analysis tools that can be used to optimize potential 

designs for enhanced structural performance. This approach 

has been used in other railroad applications; specifically in the 

development of crush zones for passenger equipment [4-6], a 

state-of-the-art cab car end frame design [7], and improved 

locomotive crashworthiness features [8]. 

In the present application for railroad tank cars, this 

systematic approach entails the following steps: (1) defining 

the collision conditions of concern, (2) evaluating the structural 

behavior of conventional tank car designs under these 

collisions conditions, and (3) developing and evaluating 

alternative designs that may improve performance. In this 

context, improvement means increasing the survivable impact 

speed under accident loading conditions over that of 

conventional or current tank car designs. Evaluations of the 

alternative designs are accomplished iteratively. 

The approach used in conducting this research is 

summarized in the flow diagram illustrated in Figure 1. 

Moreover, the figure is a schematic representation of the 

scientific method in which an alternative design is 

hypothesized to provide improvement over existing designs. 

Evaluation tools are then used to confirm or refute the 

hypothesis. In the present work, the evaluation techniques or 

tools are generally numerical models that have been verified 

and validated with experimental data. 

Figure 1: Framework for Improved Tank Car Design 

Development 

Define collision conditions of concern

Accident scenarios involving hazmat release from tank 

cars have been developed from review of accidents, and are 

intended to bound the range of accidents that can lead to 

release of hazmat from a tank car. For example, each of the 

accidents listed in Table 1 occurred from either a derailment or 

a train collision. In addition, studies of accident data, which 

include the FRA’s Railroad Accident/Incident Reporting 
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3 

System (RAIRS) [9], indicate that these two types of accidents 

account for a vast majority (more than 90 percent) of all 

hazmat-related accidents [10]. 

Accident data are also collected by the Railroad Tank Car 

Safety Research and Test Project, which is sponsored by the 

Railway Supply Institute (RSI) and the Association of 

American Railroads (AAR). Between 1965 and 2005, a total of 

252 tank cars released toxic inhalation hazard (TIH) materials 

in 176 accidents [11]. Figure 2 shows the distribution of these 

releases in terms of causes and quantity of released commodity. 

While less than half of all releases are caused by failures in the 

head and the shell of pressurized tank cars, such failures 

account for 85 percent of the total gallons of lost commodity. 

Failures to valves and fittings account for about one-third of 

the total accidents-caused releases, but less than 5 percent of 

the total gallons of lost lading. That is, the frequency of head 

and shell failures is slightly less than all other causes combined, 

but their consequence in terms of gallons of lost commodity is 

much greater. 

Cause of Lading Loss Gallons Lost

Head

23%

Shell

27%

Valves &

Fittings

27%

Other

23%

Head

36%

Shell

45% Other

18%

Valves & Fittings

1%

Figure 2: Accident-Caused Releases in TIH Tank Cars, 

1965-2005 [11] 

Accident history suggests that puncture can occur 

anywhere on the commodity tank, but punctures are more 

likely to occur below the beltline and near the ends of the car. 

In addition, collision dynamics modeling performed by the 

Volpe Center [12-13] suggests that the gross motions of rail 

cars in derailments and collisions produce secondary car-to-car 

impacts that are similar to those observed in actual accidents 

when cars come off the rails. Modeling of derailments and 

collisions as well as field experience suggests that the car-tocar impact scenarios of concern are head and shell impacts. 

Evaluate structural behavior of current designs

Tools have been developed and are under continuous 

refinement to evaluate different tank car designs. Development 

and refinement of these tools entail engineering analyses and 

testing. The analyses must account for the key physical aspects 

of collisions which include: (1) quantifying forces and 

deformations as functions of time which define the forcedeformation or force-indentation characteristic, (2) elasticplastic stress-strain behavior with large deformations, (3) fluidstructure interaction, and (4) material failure. For example, 

detailed finite element analysis modeling of a conventional 

chlorine tank car under generalized head [14] and shell [15] 

impacts have been conducted. Full-scale shell impact testing of 

the conventional chlorine tank car [16] was funded and 

conducted by the NGRTC project to validate the detailed FEA 

model.1

 Moreover, the detailed FEA model was used to 

simulate the full-scale shell impact test included material 

failure. The material failure model was based on benchmark 

testing of unnotched Charpy bars [17]. 

The essence of these analysis and testing efforts is shown 

in Figure 3 in which structural behavior under impact loading is 

described in terms of the force-indentation characteristic. The 

figure compares the calculated characteristic with that derived 

from the processed data from the full-scale shell impact test. 

The test was conducted with a ram car weighing 286,000 lb, 

striking the side of a conventional chlorine tank car at an 

impact speed of 15 mph, and puncturing the shell. The 

commodity tank was pressurized to 100 psi, and contained fluid 

lading with 11 percent outage. Based on the impact test data, 

the energy to puncture, or shaded area under the forceindentation curve corresponding to the test, is about 0.9 million 

ft-lb. Energy to puncture is a metric of structural performance 

that may be used to evaluate different tank car designs. 

Figure 3: Force-Indentation Characteristics for Full-Scale 

Shell Impact Test at 15 mph 

Semi-empirical evaluations of head tests performed in the 

1970s indicate that internal pressurization of the commodity 

tank decreases the velocity at which puncture is expected to 

occur [18]. Detailed finite element analyses conducted in 

support of the NGRTC project [19] also suggest that the energy 

to puncture decreases with increasing internal pressure. 

 

1

 FRA coordinated with the Volpe Center to design of the full-scale shell impact 

tests. 
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4 

Develop alternative designs

In the systematic approach adopted for this work, concepts 

for improved crashworthiness designs are formulated through 

the following process: (1) defining the desired performance, 

(2) developing strategies that are effective in meeting this 

performance, (3) and developing the tactics for implementing 

the strategies. The tactics and concept are evolved with 

engineering design studies, until a design satisfying all of the 

design requirements is developed. Design requirements 

include service, manufacturing, maintenance, repair, and 

inspection requirements, as well as crashworthiness 

performance requirements. In previous work, the desired 

performance goal was set to improve the impact energy to 

puncture the commodity tank by a factor of four over the 

conventional chlorine tank car. 

Previous research identified four design functions that can 

help maintain the structural integrity of the commodity tank 

during collisions: (1) blunt the impact loads, (2) absorb 

collision energy, (3) provide a strong foundation for the loadblunting and energy-absorbing features, and (4) control the 

load path to the commodity tank. 

Head shields on current cars principally act to blunt the 

impact load. In effect, the shield makes the size of the 

impacting object appear larger, spreading load over a greater 

area of the commodity tank. By doing so, the energy required 

to puncture the tank is increased. The addition of an energyabsorbing component can further increase the energy to 

puncture the tank. In essence, the energy absorption decreases 

the speed of impact experienced by the commodity tank. 

Strengthening the tank allows an energy-absorbing 

component to crush at a higher load. If the service loadbearing structure is separate from the commodity tank, the tank 

does not experience cyclic service loads and the potential for 

failure by fatigue cracking is reduced. 

Engineered sandwich structures have the potential to 

perform some of these functions in the conceptual design.2

 The 

primary advantages of sandwich panels over alternative 

construction materials and technologies are the ability to 

support loads in the plane of the panel while offering effective 

energy-absorbing capability in the normal (out-of-plane) 

direction. 

Sandwich panels comprise two face sheets that are 

separated by a core. A sandwich panel with a square egg-crate 

core is shown in Figure 4. The egg-crate geometry is used here 

as an exemplar. A variety of core geometries are available to be 

used within sandwich structures. The separation of the face 

sheets by the core increases the moment of inertia of the panel 

which produces a higher bending stiffness-to-weight ratio than 

solid plates. The mechanical behavior of sandwich structures 

depends on the properties of the face sheet and core materials, 

the configuration of the core, and attachments. The face sheets 

 

2

 The term engineered is used to emphasize that the structure requires careful 

design to achieve desired performance levels. 

and the core of the sandwich structures described in this paper 

are steel. 

Figure 4: Annotated Egg-Crate Sandwich Panel 

PRELIMINARY CONCEPTUAL DESIGN 

A preliminary conceptual design that integrates the four 

functions was developed and described in previous work [3]. 

This alternative design treats the conventional tank as a 

protected entity which is encased by a separate carbody. The 

conventional tank is reinforced with stiffeners around the head. 

The reinforced tank with insulation sits within a structural 

carbody, which is made of sandwich panels. Between the 

reinforced and insulated tank and the exterior carbody is a 

structural foam saddle, designed to support the tank while 

isolating it from service loads from the carbody. Therefore, the 

construction sequence for this design is a three-phase 

fabrication process: (1) construction of the reinforced tank, (2) 

construction of the exterior carbody, and (3) marriage of the 

reinforced tank and carbody. 

Figure 5 shows the various stages of assembly for an 

alternative design. This illustration differs from that shown in 

the original [3] in two ways. Here only the ends (head) of the 

conventional tank are reinforced. The other difference is that 

the sandwich panels comprising the exterior carbody structure 

in Figure 5 are curved, whereas flat panels forming an 

octagonal cross-section were considered in the original 

concept. 

Figure 5: Progression of Conceptual Design Assembly 
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5 

The commodity tank is assumed to have a wall thickness 

of 5/8 inch and a nominal capacity of 17,300 gallons. The 

entire car, when fully loaded, weighs less than 286,000 lb. 

With this weight budget, 39,000 lb of material is available for 

new structure that may be applied for safety enhancements. 

The extreme width of the exterior carbody is 128 inches. The 

roof/top of the carbody is about 131⁄2 feet above top-of-rail. 

Moreover, these dimensions are within the clearances of AAR 

Plate B [20]. 

Dynamic, nonlinear finite element analyses [21] were used 

to conduct initial evaluations of the preliminary conceptual 

design under generalized head and shell impact scenarios at a 

closing speed of 15 mph. In these impact scenarios, a rigid 

punch was assigned a weight of 286,000 lb. In addition, 

simplifying assumptions were made in the initial analyses. For 

example, the commodity tank contained no fluid lading and 

was unpressurized. Material failure modeling was not included 

in the analyses. Moreover, sandwich structures were assumed 

to have a square egg-crate core, and were modeled using a 

continuum element approximation [22-24]. Under these 

assumptions, the initial evaluations suggested that the desired 

performance goal could be attained [3]. Considering the 

generalized head impact scenario, the improved design was 

shown to absorb about eight times as much energy as the 

conventional tank. Considering the generalized shell impact 

scenario, the improved concept was shown to absorb about 4.5 

times as much energy as the conventional tank. More refined 

evaluations of the preliminary conceptual design are conducted 

by including additional details, which are described as follows. 

DETAILED CONCEPTUAL DESIGN EVALUATION 

The work in progress described in this paper includes 

studies to examine the effect of sandwich structure details, 

refined analysis to evaluate the integrated conceptual design, 

and an assessment of the continuum approach to modeling 

sandwich structure cores. 

Effect of Sandwich Structure Details

A variety of sandwich structure details can satisfy the 

allowable weight and space budgets for the alternative design. 

The influence of selected sandwich structure parameters on the 

force-indentation characteristic is examined. Flat sandwich 

panels with square egg-crate cores are assumed. Specifically, 

the effects of two parameters are studied: core web thickness 

and sandwich height. The sandwich panels are assumed to be 

made from TC-128B tank car steel. 

Figure 6 shows the effect of varying the core web 

thickness. In these results, the face sheet thickness is held 

constant and equal to 0.125 inch. The figure indicates that the 

force levels increase with increasing core web thickness. 

The results shown in Figure 7 suggest that a range of 

sandwich heights can provide similar performance in terms of 

the force-indentation characteristic. Face sheet and core 

thicknesses are varied to maintain a constant sandwich weight 

per unit area. The figure also shows that the force-indentation 

characteristic for these three sandwich structures is equivalent 

to that of a solid plate with a thickness of 1.7 inches. 

Figure 6: Influence of Core Web Thickness 

Figure 7: Influence of Sandwich Height 

Refined Analysis

Numerical evaluations of the preliminary conceptual 

design are refined by including additional details. For 

example, both the exterior carbody and the commodity tank are 

included to examine the deformation of these components in 

the integrated design during a collision. In the initial 

evaluations [3], the protection to the commodity tank offered 

by the exterior carbody was modeled by an unconstrained, flat 

sandwich panel. In the refined analysis, the commodity tank is 

pressurized but contains no fluid lading. In addition, material 

failure is not explicitly modeled. Evaluations based on these 

assumptions provide estimates of the energy absorption 

demands for the integrated conceptual design. 

Figure 8 shows schematic diagrams of the generalized 

impact scenarios for which the evaluations of the integrated 

design are performed. In both scenarios, the tank car is 

impacted by a rigid punch with a 6-inch by 6-inch face. 
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6 

Figure 8: Impact Scenarios for Integrated Design 

Evaluation 

Table 2 summarizes the details of the sandwich structures 

that make up the exterior carbody, and are assumed in the 

evaluation of the integrated design. The particular details are 

annotated in Figure 4. 

Table 2: Details for Integrated Design Evaluation 

Detail Impact Scenario 

Shell Head 

Sandwich Height 6 inches 6 inches 

Core Web Thickness 0.25 inch 0.1 inch 

Face Sheet Thickness 0.125 inch 0.25 inch 

Considering the generalized head impact scenario, the bar 

graph shown in Figure 9 compares three different energies: (1) 

the kinetic energy associated with a mass corresponding to 

286,000 lb and an impact speed of 30 mph (8.6 million ft-lb), 

(2) the energy to puncture a conventional chlorine tank (about 1 

million ft-lb), and (3) the energy absorbed by different 

components in the integrated design, the sum of which is equal 

to the kinetic energy. That is, the kinetic energy is converted 

into different forms of energy in the integrated design: (1) 

energy dissipated in plastic deformation of the commodity tank, 

(2) energy dissipated in plastic deformation of the exterior 

carbody, (3) energy dissipated by the reinforcements, and (4) 

work against the internal pressurization which is referred to as 

fluid work. The figure indicates that most of the energy (over 

60 percent) is dissipated in plastically deforming the exterior 

carbody. Consequently the carbody may be considered as a 

sacrificial structure. For a given impact scenario, the tank will 

maintain its integrity if the combined energy absorption 

capacity of the tank and the sacrificial structure is higher than 

the kinetic energy. The more kinetic energy that gets absorbed 

in the sacrificial structure, the less kinetic energy has to be 

absorbed by the commodity tank. Therefore, in a higher speed 

impact, the tank will have a greater capacity to absorb kinetic 

energy than if no sacrificial structure were present. 

Kinetic Energy Conventional Head Improved Head

Energy (Million ft-lb)

0

1

2

3

4

5

6

7

8

9

10

Tank

20%

Carbody

61%

Fluid Work

8%

Reinforcements

11%

Figure 9: Energy Absorption in Generalized Head Impact 

The energy to puncture the conventional chlorine tank car 

is calculated from finite element analyses that include the effect 

of fluid-structure interaction by modeling the fluid lading with 

a Lagrangian mesh [14]. In addition, material failure is 

included in the analyses by assuming that failure initiates when 

the effective plastic strain exceeds a critical value for a given 

state of stress or stress triaxiality [25]. 

Similarly, Figure 10 compares energies for the generalized 

shell impact at 25 mph. In this impact scenario, the kinetic 

energy is equal to 6 million ft-lb, and the energy to puncture the 

conventional shell is 1.2 million ft-lb. In the original 

preliminary conceptual design reported in [3], the commodity 

tank was reinforced with sandwich panels to provide a strong 

foundation for the exterior carbody structure to blunt the 

impact load and absorb collision energy in the event of a shell 

impact. When the reinforcement is included in the analysis, 

however, its contribution to the total energy absorption is 

nearly negligible. Figure 10 shows that without including 

reinforcements, the sacrificial structure (i.e., exterior carbody) 

can potentially absorb over 70 percent of the kinetic energy. 

Refined evaluations have quantified the energy-absorption 

demands of the preliminary integrated design. Based on these 

results, the impact energy to puncture the commodity tank is 

estimated to be about six to nine times that over the 

conventional chlorine tank car. In the shell impact scenario, a 

relatively stiff sandwich structure facilitates energy dissipation 

over a large volume of the carbody. In the head impact 

scenario, reinforcements help to activate energy dissipation 

over a significant volume of the head sandwich as well as the 

tank. However, success of this stiffening concept relies on the 

ability of the external structure to effectively blunt the 

impactor. Localization of stress and deformation restricts the 

volume of material available to dissipate significant energy. 
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7 

Kinetic Energy Conventional Shell Improved Shell

Energy (Million ft-lb) 0

1

2

3

4

5

6

7

8

Tank

12%

Carbody

72%

Fluid Work

16%

Figure 10: Energy Absorption in Generalized Shell Impact 

Discrete Modeling versus Continuum Approach

Initial evaluations of the preliminary conceptual design 

reported in [3] and the results presented in this paper are based 

on the continuum approximation [22-24] of the sandwich core. 

In order to assess these evaluations, the continuum approach to 

modeling the core geometry of square egg-crate sandwich 

panels is examined. 

Figure 11 compares the force-indentation characteristic for 

a flat sandwich panel with a square egg-crate core calculated 

using the continuum element approach with that calculated 

from a discrete finite element model. In the continuum model, 

the core is modeled using two layers of solid elements with 

properties selected to match the deformation of the square eggcrate geometry in shear and compression [23-24]. In the 

discrete model, the core is modeled with shell elements. In 

both models, the face sheets are modeled with shell elements. 

Referring to Figure 4, these results are based on the following 

sandwich panel details: 6-inch web spacing, 6-inch sandwich 

height, 0.25-in face sheet thickness, and 0.125-in core web 

thickness. In addition, the entire sandwich panel is made from 

TC-128B tank car steel. Moreover, the force-indentation 

characteristic provided by the continuum modeling approach 

for this particular sandwich configuration agrees very closely 

with that from the discrete model up to about 15 inches of 

indentation. Consideration is being given to alternative core 

geometries since determining that the egg-crate core sandwich 

panel force would start to decrease after 15 inches of 

displacement in the shell impact. Core geometries which 

potentially allow greater displacements are currently being 

evaluated. 

The continuum element approach is useful to perform 

rapid evaluations, but a discrete model of the core appears to be 

necessary in order to capture local effects and examine failure 

accurately. Continuum elements are appropriate to model the 

behavior of sandwich structures at locations sufficiently far 

away from the point of impact. 

Indentation (inches)

0 5 10 15 20 25 30

Force (kips)

0

200

400

600

800

1000

1200

1400

1600

Continuum element

Discrete model

Figure 11: Comparison between continuum approach and 

discrete model 

Fabrication Evaluation

In principle, it may be desirable for the steels used in the 

fabrication of sandwich structures to possess high strength. In 

practice, it is also desirable for such steels to be easily welded 

and readily formable to create curved shapes when necessary 

(which implies high ductility). In addition processes to join 

components together (such as welding and bolting) require 

study for the present application of sandwich structures. 

The FRA has awarded a Phase III Small Business 

Innovative Research (SBIR) contract to Cellular Materials 

International, Inc. (CMI) to study the fabrication issues of 

material selection and joining processes regarding sandwich 

structures for railroad tank car applications.3

 Moreover, the 

primary task for CMI in this work is to build sandwich 

structure specimens that will be used in severe deformation 

tests that are discussed in the following section. 

DISCUSSION 

Component-level impact tests were conducted on sandwich 

structures designed to protect a tank head conducted under the 

NGRTC project. Two types of sandwich structures were tested; 

one with a foam-core, the other was an engineered metal 

structure (EMS) with corrugated/pleated core geometry. 

Neither of these test articles, however, was optimized to 

maximize structural performance. Consequently, the EMS with 

corrugated/pleated core appeared to have inferior performance 

to that of an equivalent solid plate [19]. This behavior was also 

observed for stiff honeycomb-core panels developed to resist 

blast loads for ships hulls for the Office of Naval Research 

[26]. In the naval application, the honeycomb panel is superior 

to a solid plate when subjected to a planar impulse, but inferior 

when more localized. By analogy, the EMS with the 

 

3

 Phase III refers to the commercialization stage of the SBIR. Phase I and II 

refer to feasibility assessment and developmental stages of the innovation. CMI 

examined sandwich panels for naval applications in Phases I and II of the 

SBIR. 
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8 

corrugated/pleated core may be superior to an equivalent solid 

plate in certain impact scenarios (e.g., those involving 

relatively large indenters), but has been shown to be inferior for 

impacts with a 6-in by 6-in indenter. 

An alternative core for the naval application, called a 

doubly-corrugated soft core [26], was found to provide 

superior performance to an equivalent solid plate under planar 

and localized impulses. Again, by inference, it would appear 

reasonable that an alternative core (to be determined) may 

outperform a solid plate over a greater range of indenter sizes 

for the tank car application. 

In analyses conducted in support of the NGRTC project 

[19], it was estimated that a solid steel plate of about three 

inches would be required to prevent puncture from a side 

impact assuming a rigid indenter with a 6-in by 6-in face that 

weighs 295,000 lb and a closing speed of 25 mph. Under the 

same assumptions, a solid steel plate more than six inches thick 

would be required to prevent a head impact at a closing speed 

of 30 mph. These equivalent thickness estimates are based on a 

shear failure criterion originally developed through research 

sponsored by the industry in the 1970s [18]. In this criterion, 

the failure mechanism initiating puncture is excessive shear 

around the perimeter of the indenter.4

In the context of design strategies, solid steel plates with 

increased thickness would provide sufficient load blunting but 

little energy absorption capability. In addition, increased 

thickness can lead to a relatively stiff structure that localizes the 

impact forces, which in turn can lead to localized material 

failure as opposed to large structural denting. 

Generally speaking, sandwich structures are used in 

applications that require strong bending strength. In the current 

application, however, protection of the commodity tank against 

penetration offered by sandwich structures is realized through 

blunting of impact loads and absorption of the collision energy. 

Refined evaluations have shown that the energy absorption 

demands on the sandwich structures in the preliminary 

integrated design to prevent puncture are challenging. The 

conversion of kinetic energy into plastic deformation depends 

on many factors including the magnitude of the impact load, 

mode of deformation and failure, and material properties. As 

seen in the naval application, a relatively soft core may be more 

desirable over a stiffer core in order to enable load blunting and 

energy absorption more effectively. The complexities in 

designing sandwich structures are manifested through a 

competition of failure modes prior to penetration and failure of 

the commodity tank. These failure modes of the sandwich 

structure include tearing of the face sheets, crushing or 

buckling of the core, and shear failure of the core. In the event 

of a collision, for example, high stress concentrations at the 

edges of the indenter can cause a large differential in the 

displacement of the face sheets. Tension in the face sheets 

makes them vulnerable to tearing or failure in this situation. 

 

4

 The shear failure criterion was used to develop semi-empirical equations to 

predict head punctures [16]. 

Moreover, the specific technical challenge in future work will 

be to develop a deformable energy-absorbing structure with an 

equivalent thickness less than three to six inches. 

Alternative core geometries to the egg-crate core geometry 

are shown in Figure 12. Severe deformation tests are being 

planned to examine load-deformation behavior of these core 

geometries. The pipe or tubular core has been considered in 

automobile applications as car bumpers and guardrails [27]. 

The X-core is similar to the doubly-corrugated core developed 

for the naval application [26]. The diamond core is a variation 

of an alternative core considered for the naval application [28]. 

The severe deformation tests will be used to: (1) rank the 

different core geometries for structural performance and (2) 

confirm the modeling of sandwich structures under the 

prescribed loading conditions. 

Pipe Core 

X-Core 

Diamond Core 

Figure 12: Candidate Core Geometries 

Initial results from ongoing analyses suggest that the core 

arrangements shown in Figure 12 potentially allow greater 

displacements in the shell impact than the egg-crate core. 

Some of these analyses results also indicate that the boundary 

conditions of the panel – the manner in which the panel is 

supported – significantly influence the performance. 

SUMMARY AND NEXT STEPS 

Recent accidents involving the release of hazmat, with 

consequent fatalities, have focused attention on the structural 

integrity and crashworthiness of railroad tank cars. 

Government-sponsored research is ongoing to understand the 

load-deformation behavior of tank cars during accidents as well 

as during normal operating conditions. Research will be 

conducted to understand structural performance of tank cars in 

the current fleet. A mechanical engineering design approach 

that relies on the scientific method was reported in previous 

work, and is being applied to develop alternative designs and 

technologies to maintain tank integrity in all scenarios. An 

alternative concept that treats the pressurized commoditycarrying tank as a protected entity has been developed from this 

approach. Moreover, this alternative concept is being evaluated 

for structural performance, and is being studied for feasibility 

in fabrication. Other alternative designs may be considered. 
This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited. 

9 

Ultimately, testing of optimized designs must be conducted 

to validate the modeling and the feasibility of sandwich 

structures for railroad applications. Such tests are under 

development. Component-level tests are being planned to 

verify the modeling of critical and difficult-to-analyze 

components. Tank car designs will be refined from results of 

detailed analysis and testing. 

ACKNOWLEDGEMENTS 

The work described in this paper was sponsored by FRA 

Office of Research and Development. Dr. Thomas Raslear is 

the Acting Chief of the Equipment and Operating Practices 

Division. Mr. Francisco Gonzalez is the FRA program 

manager for research on railroad tank cars. 

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